<html>
<head>
  <style>
  
  div#test 
  {
    border:10px solid black;
    background:gold;
  }
  div#test:hover 
  {
    border:20px solid blue;
    transition: border(linear,0.5s);
  }
  
  </style>  
</head>
<body>
  <div #test>This div will acquire a slightly thicker blue border when hovered.</div>
</body>
</html>